I have a friend, and she's so allergic to milk, that once, when her dad kissed her on her cheek, after drinking a coffee with cream, her cheek got all rashy in 5 minutes. And Once her mom made her and her friend sandwiches. The mom made one with cheese and one without, but she used the same knife. So, even though the mom wiped most of the cheese off before she cut her daughter's sandwhich, just that tiny amount of cheese grease made her throw up when she ate it.
I'm just glad I'm not lactose intolerant.